Size-dependent visible-light-enhanced Cr(VI) bioreduction by hematite nanoparticles

Light irradiation would affect the electron transfer between dissimilatory metal-reducing bacteria (DMRB) and semiconducting minerals, which may impose a great influence on the biogeochemistry cycle of heavy metals. However, the size effect of semiconducting minerals on the its electron transfer with DMRB and microbial Cr(VI) reduction under visible light irradiation is little known. Herein, the Cr(VI) reduction by Shewanella oneidensis MR-1 (MR-1) was investigated in the presence of hematite nanoparticles with average diameters of 10 nm and 50 nm in dark and under visible light irradiation. It is found that hematite nanoparticles adhered onto MR-1 cells to form the composites, leading to the decrease in surface sites and Zeta potential. Hematite mediated-Cr(VI) bioreduction rate under visible light irradiation was 0.342 h^-1, which is 3.4 folds enhancement compared with that in dark and 4.4 folds compared with the MR-1 alone under visible light irradiation. Decreasing nanoparticle size of hematite from 50 nm to 10 nm promoted the Cr(VI) reduction under visible light irradiation but impeded it in dark. It was deduced that the bioelectrons from MR-1 could promote the separation of photoelectron-hole pairs of light-irradiated hematite, which consequently enhanced the Cr(VI) bioreduction by MR-1-hematite composites. Moreover, mutant strains experiments demonstrated the vital role of c-cytochrome for the conducting network actively established by MR-1 with hematite nanoparticles. Those findings expand the understanding of the electron transfer pathway for enhancing Cr(VI) reduction by hematite-MR-1 composites, and the impact of particle size on the interaction between semiconducting mineral and electroactive bacteria under light irradiation.